It has a 0-10v dim input. Can I just take a 10k digital pot and dim away?

Probably... But, it would be helpful to know the input impedance (or current).

For example, if the input impedance (of the dimmer circuit) is 1K and you use a 10k pot, the voltage won't be linear (you'll get around 2V at the mid-point, instead of 5V). To some extent, you can compensate for non-linearity in the software if necessary.

And, if the impedeance is really low (very unlikely) you can get excess current through the pot, and damage it.

It wouldn't hurt to try with a regular analog pot. Worst case you fry the pot, but that's very unlikely.

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Are digital pots fast enough for "switching" as well?

Probably... What frequency are we talking about? You can check the specs, but I assume they work at audio frtquencies (beyond 20khz).

Digital pots tend to be very fast, fast enough that your eyes wouldn't be able to respond as quickly as they can switch. It's unclear what the input requirements are for your selected dimmer; I could find the information easily from the site you listed. My guess is that the input to the dimmer is high impedance (as in a MOSFET gate), and that the NPN controller you showed a scematic for would be just fine if all you want to do is switch on and off. You could also use a MOSFET for the transistor and virtually elimate the load from the Arduino (presumably the source of your 0-5V pulse).

Digital pots tend to be very fast, fast enough that your eyes wouldn't be able to respond as quickly as they can switch. It's unclear what the input requirements are for your selected dimmer; I could find the information easily from the site you listed. My guess is that the input to the dimmer is high impedance (as in a MOSFET gate), and that the NPN controller you showed a scematic for would be just fine if all you want to do is switch on and off. You could also use a MOSFET for the transistor and virtually elimate the load from the Arduino (presumably the source of your 0-5V pulse).

Hi thanks for your reply, but it turns out I have to generate PWM not resistance, please read the update above.